This invention relates to new process for removing heavy metal ions with an improved adsorbent possessing high mechanical strength and to a process for producing such adsorbent.
Among the methods known hitherto for treatment of industrial effluents containing heavy metals, the most popular is one involving neutralization with lime. This neutralization method comprises adding quick lime or calcium carbonate to such industrial effluents thereby neutralizing any acidic substance in the effluents and at the same time converting heavy metals contained therein into easily precipitatable hydroxides. However, this method is limited in the kind and quantity of heavy metals capable of being separated and fails to attain complete removal of such heavy metals as mercury and lead. Moreover, this method results in a large amount of a heavy metal-containing sludge that is difficult to treat, thus causing secondary environmental pollution.
In addition to this method, there has been proposed a method utilizing active carbon, an ion-exchange resin or a chelate-forming agent for complexing heavy metals in effluents. However, such methods are unsuited for treating extremely large volumes of liquids, such as industrial effluents, as they require expensive treating agents and recovery or regeneration of the treating agents is difficult.
On the other hand, it is known that nitrohumic acid exhibits high adsorptivity for heavy metals dissolved in water and is easily regenerated by the action of an acid. However, this compound is low in mechanical strength when shaped into granules and is finely pulverized under even slight pressure when charged into a packing tower. In addition, this compound has the disadvantage of partially dissolving when used for treating an alkaline or neutral solution, resulting in coloration of the solution in yellowish brown. If this compound is incorporated with a conventional binder to enhance mechanical strength, the excellent heavy metal adsorbability inherent to nitrohumic acid suffers. Therefore, nitrohumic acid has not yet had practical use as a heavy metal adsorbent for the treatment of waste effluents, notwithstanding the cheapness of this compound.